Same problem exists for the USB bus. The mini supplies power to the bus while asleep. People have gotten around that by using a USB hub with a power selector switch for USB. For firewire, the only thing I can think of is to cut the power wire in the cable and use a relay to close the circuit when the car is turned on.
I don't know if that would make it difficult to wake up the sound card or if it would cause some kind of crash to the Mini, though.